here im telling you how you can tune a warthog (or every other vehicle) in Halo 1 for pc
REMEBER: always backup your maps for in case your doing something wrong!!!!

Step 1: Download and Backup Some Stuff

first download a few things

Halo Map Tools: you can download it here
Halo Map Swithcer: you can download it here

Step 2: Backup

open up Halo Map Switcher and clikck on "Backup All Maps"
if you did something wrong and want the original stuff back open up HMS and click on "Restore All Maps"

Step 3: Tune Your Warthog

Now were going to tune the Warthogs speed. Open up Halo Map Tools (HMT) and click in the left upper corner on "file" and "open", now open up the map Bloodgulch (you should search in the Halo map). i did this because im sure it is in here =p
now youre opened it you see a list, scroll down to Vehicles and click on vehicles\warthog\mp_warthog
On the right side you see some "parameters". Set Top Speed (forward) to 0,8 (put a , not a . cause if you do a . it won't work) Set Top Speed (Reverse) to 0,4 set Acceleration to 0,0099 and Left and Right Turn Angle to 25 and -25
Now search for Physics in the list where you clicked Vehicles. Choose vehicles\warthog\warthog
set Gravity Multiplier to 2 or less.
exit HMT (you dont have to save anything)

Now start Halo and choose Multiplayer and choose for the map Bloodgulch, get in a Warthog and if you done right enjoy =)